Physics: Principles with Applications, 7th edition ConcepTest PowerPoints Chapter 18 Physics: Principles with Applications, 7th edition Giancoli © 2014 Pearson Education, Inc. This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials.

ConcepTest 18.1 Connect the Battery Which is the correct way to light the lightbulb with the battery? d) all are correct e) none are correct

ConcepTest 18.1 Connect the Battery Which is the correct way to light the lightbulb with the battery? d) all are correct e) none are correct Current can only flow if there is a continuous connection from the negative terminal through the bulb to the positive terminal. This is only the case for (c).

ConcepTest 18.2 Ohm’s Law You double the voltage across a certain conductor and you observe the current increases three times. What can you conclude? a) Ohm’s law is obeyed since the current still increases when V increases b) Ohm’s law is not obeyed c) This has nothing to do with Ohm’s law

Follow-up: Where could this situation occur? ConcepTest 18.2 Ohm’s Law You double the voltage across a certain conductor and you observe the current increases three times. What can you conclude? a) Ohm’s law is obeyed since the current still increases when V increases b) Ohm’s law is not obeyed c) This has nothing to do with Ohm’s law Ohm’s law, V = I R, states that the relationship between voltage and current is linear. Thus for a conductor that obeys Ohm’s Law, the current must double when you double the voltage. Follow-up: Where could this situation occur?

ConcepTest 18.3a Wires I a) dA = 4 dB b) dA = 2 dB c) dA = dB d) dA = 1/2 dB e) dA = 1/4 dB Two wires, A and B, are made of the same metal and have equal length, but the resistance of wire A is four times the resistance of wire B. How do their diameters compare?

ConcepTest 18.3a Wires I a) dA = 4 dB b) dA = 2 dB c) dA = dB d) dA = 1/2 dB e) dA = 1/4 dB Two wires, A and B, are made of the same metal and have equal length, but the resistance of wire A is four times the resistance of wire B. How do their diameters compare? The resistance of wire A is greater because its area is less than wire B. Since area is related to radius (or diameter) squared, the diameter of A must be two times less than B.

ConcepTest 18.3b Wires II a) it decreases by a factor 4 b) it decreases by a factor 2 c) it stays the same d) it increases by a factor 2 e) it increases by a factor 4 A wire of resistance R is stretched uniformly (keeping its volume constant) until it is twice its original length. What happens to the resistance?

ConcepTest 18.3b Wires II a) it decreases by a factor 4 b) it decreases by a factor 2 c) it stays the same d) it increases by a factor 2 e) it increases by a factor 4 A wire of resistance R is stretched uniformly (keeping its volume constant) until it is twice its original length. What happens to the resistance? Keeping the volume (= area x length) constant means that if the length is doubled, the area is halved. Since , this increases the resistance by four.

ConcepTest 18.4 Dimmer a) the power b) the current c) the voltage d) both a) and b) e) both b) and c) When you rotate the knob of a light dimmer, what is being changed in the electric circuit?

Follow-up: Why does the voltage not change? ConcepTest 18.4 Dimmer a) the power b) the current c) the voltage d) both a) and b) e) both b) and c) When you rotate the knob of a light dimmer, what is being changed in the electric circuit? The voltage is provided at 120 V from the outside. The light dimmer increases the resistance and therefore decreases the current that flows through the lightbulb. Follow-up: Why does the voltage not change?

ConcepTest 18.5a Lightbulbs Two lightbulbs operate at 120 V, but one has a power rating of 25 W while the other has a power rating of 100 W. Which one has the greater resistance? a) the 25 W bulb b) the 100 W bulb c) both have the same d) this has nothing to do with resistance

ConcepTest 18.5a Lightbulbs Two lightbulbs operate at 120 V, but one has a power rating of 25 W while the other has a power rating of 100 W. Which one has the greater resistance? a) the 25 W bulb b) the 100 W bulb c) both have the same d) this has nothing to do with resistance Since P = V2 / R the bulb with the lower power rating has to have the higher resistance. Follow-up: Which one carries the greater current?

ConcepTest 18.5b Space Heaters I Two space heaters in your living room are operated at 120 V. Heater 1 has twice the resistance of heater 2. Which one will give off more heat? a) heater 1 b) heater 2 c) both equally

ConcepTest 18.5b Space Heaters I Two space heaters in your living room are operated at 120 V. Heater 1 has twice the resistance of heater 2. Which one will give off more heat? a) heater 1 b) heater 2 c) both equally Using P = V2 / R, the heater with the smaller resistance will have the larger power output. Thus, heater 2 will give off more heat. Follow-up: Which one carries the greater current?